RR is not supported on the CX3. Also, the CX3 is not supported at all with vSphere 5.
The CX4 does support RR with the right FLARE code revision (29 and above) and initiator settings (Failover Mode 4)
However - dont forget that round robin DOESN"T use multiple paths simulataneously - it just switches rapidly between them. If you want to use multiple paths *at the same time*, you need powerpath.